An 8.5-kb segment of the PMP22 promoter responds to loss of axon signals during Wallerian degeneration, but does not respond to specific axonal signals during nerve regeneration

Altered expression of the PMP22 gene causes Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP). We have examined the promoter activity of 8.5 kb upstream of the first coding exon of the rat peripheral myelin protein-22 (rPmp22) gene in transgenic mice. We found that the -8.5 kb rPmp22/chloramphenicol acetyl transferase (CAT)/beta-galactosidase (lacZ) construct directs reporter gene expression in a weakly developmental and tissue-specific pattern, consistent with the expression pattern of the endogenous Pmp22 gene. The -8.5 kb rPmp22/CAT/lacZ transgene responds to loss of axonal signals during Wallerian degeneration but unlike the endogenous Pmp22 gene, the transgene fails to respond to axonal signals during nerve regeneration after a sciatic nerve crush injury. In conclusion, the function of the -8.5 kb rPmp22/CAT/lacZ transgene suggests that there are separable regulatory elements in the rPmp22 gene that respond differently to axonal signals received by Schwann cells during nerve development, and during remyelination.

A combinatorial network of evolutionarily conserved myelin basic protein regulatory sequences confers distinct glial-specific phenotypes

Myelin basic protein (MBP) is required for normal myelin compaction and is implicated in both experimental and human demyelinating diseases. In this study, as an initial step in defining the regulatory network controlling MBP transcription, we located and characterized the function of evolutionarily conserved regulatory sequences. Long-range human-mouse sequence comparison revealed over 1 kb of conserved noncoding MBP 5' flanking sequence distributed into four widely spaced modules ranging from 0.1 to 0.4 kb. We demonstrate first that a controlled strategy of transgenesis provides an effective means to assign and compare qualitative and quantitative in vivo regulatory programs. Using this strategy, single-copy reporter constructs, designed to evaluate the regulatory significance of modular and intermodular sequences, were introduced by homologous recombination into the mouse hprt (hypoxanthine-guanine phosphoribosyltransferase) locus. The proximal modules M1 and M2 confer comparatively low-level oligodendrocyte expression primarily limited to early postnatal development, whereas the upstream M3 confers high-level oligodendrocyte expression extending throughout maturity. Furthermore, constructs devoid of M3 fail to target expression to newly myelinating oligodendrocytes in the mature CNS. Mutation of putative Nkx6.2/Gtx sites within M3, although not eliminating oligodendrocyte targeting, significantly decreases transgene expression levels. High-level and continuous expression is conferred to myelinating or remyelinating Schwann cells by M4. In addition, when isolated from surrounding MBP sequences, M3 confers transient expression to Schwann cells elaborating myelin. These observations define the in vivo regulatory roles played by conserved noncoding MBP sequences and lead to a combinatorial model in which different regulatory modules are engaged during primary myelination, myelin maintenance, and remyelination.

Phenotypic differences between peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) mutations associated with Charcot-Marie-Tooth-related diseases

Mutations in the genes for peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) cause human hereditary neuropathies with varying clinical and pathological phenotypes. In this study, we examine the effects of representative disease-causing mutations on the subcellular distribution of their corresponding PMP22- and P0-enhanced green fluorescent protein (EGFP) fusion proteins. In transiently transfected HeLa and 293 cells, we find that wild-type P0-EGFP and PMP22-EGFP are efficiently synthesized and transported through the secretory pathway to the plasma membrane. The P0-EGFP and PMP22-EGFP mutants can be classified into several groups: those that are transported to the plasma membrane as in the majority of P0 mutants; those that are retained in the endoplasmic reticulum as in the majority of PMP22 mutants; and those that are a mixture of the two. In addition, several of these disease-causing mutations are associated with the development of abnormal intracellular cytoplasmic structures that we have previously identified as either intracellular myelin figures or aggresomes. Our studies indicate that different types of PMP22 and P0 mutations are associated with specific intracellular chaperone proteins, including calnexin and BiP, and that these associations can be altered by glycosylation. These findings indicate that the various P0 and PMP22 mutants may exert their pathogenic effects in different subcellular compartments and by different mechanisms in the mammalian cell.

PMP22 carrying the trembler or trembler-J mutation is intracellularly retained in myelinating Schwann cells

Missense mutations in the murine peripheral myelin protein-22 gene (Pmp22) underly the neuropathies in the trembler (Tr) and trembler-J (Tr-J) mice and in some humans with Charcot-Marie-Tooth disease. We have generated replication-defective adenoviruses containing epitope-tagged, wild-type-, Tr-, or Tr-J-PMP22 bicistronic with the Lac-Z reporter gene. These viruses were microinjected into the sciatic nerves of 10-day-old Sprague-Dawley rats and, later, analyzed by immunohistochemistry to determine the distribution of mutant protein in infected myelinating Schwann cells. We found that epitope-tagged, wild-type PMP22 is successfully transported to compact myelin, whereas the Tr and the Tr-J mutant proteins are retained in cytoplasmic compartment, colocalizing with the endoplasmic reticulum. These results provide in vivo evidence that the pathogenesis of the Tr and Tr-J mutations are most likely a function of abnormal retention within the endoplasmic reticulum of myelinating Schwann cells.

The anatomy and cell biology of peripheral myelin protein-22

The gain of function phenotypes exhibited by the heterozygous Tr, Tr-J, and CMT1A mutations indicate that these mutations interfere with more than the function of a single PMP22 allele. The identification of proteins that interact with PMP22 and that are sensitive both to stoichiometry and the effects of the mutations could provide important leads to a unified hypothesis to explain the riddle of the PMP22-related neuropathies.

Common themes in peripheral neuropathy disease genes

After a century of study, mutations in connexin32, peripheral myelin protein22, and protein zero are now known to culminate in the prototypical phenotype of Charcot-Marie-Tooth disease. Many of these mutations have been modeled in rodents and in tissue culture. Consequently, structure-function predictions for these mutations are now possible and detailed analyses of many of them are ongoing. Despite the marked differences in the functions of these three proteins, it is profitable to consider the many similarities between them, including the types of mutational mechanisms and their effects on myelin structure and function. Accordingly, the biology and genetics of Charcot-Marie-Tooth disease and other inherited peripheral neuropathies due to mutations in these proteins are reviewed.